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Сфера застосування неч^ко! логiки е надзвичайно широкою - економь ка, екологiя, медицина, техшка, сфера обслуговування тощо. Особливо! уваги заслуговуе використання даного методу в еколого-економiчнiй сферi, яка характеризуемся високою динамiчнiстю, стохастичнiстю i комплекснiстю.
Лiтература
1. Люгер, Д. Искусственный интеллект: стратегии и методы решения сложных проблем, 4-е издание/ Пер. с англ. - М.: Изд. дом "Вильямс", 2003. - 864 с.
2. Ротштейн А.П. Интеллектуальные технологии идентификации: нечеткие множества, генетические алгоритмы, нейронные сети. - Винница: "УШВЕРСУМ-Вшниця", 1999. - 462 с.
3. Рутковская Д., Пилиньский М., Рутковский Л. Нейронные сети, генетические алгоритмы и нечеткие системы/ Пер. с польск. И. Д. Рудинского. - М.: Горячая линия - Телеком, 2004. - 464 с.
4. Туниця Ю.Ю. Еколопчна Конститущя Землг 1дея. Концепщя. Проблеми. - Львiв: Вид. ц. Львiвського НУ iм. 1вана Франка, 2002. - 264 с.
5. Devjak S., Bencina J. Appraising Investment Projects in Public Administration with Fuzzy Logic// Proceedings of the 7th International Symposium on Operational Research S0R'03 in Slovenia, Podcetrtek, September 24-26, 2003.
6. 1нтелектуальш технологп DATA MINING: Неч^ка математика. [Цит. 07.11.2005 р.]. - Доступно з: <www.zlet.lviv.ua>._
Утьям Штон, Аустт Трой - факультет довктля i природныхресурав
м. Рубенштейна, Умверситет Вермонту, США
ПОеДНАННЯ ЕКОЛОГ1ЧНИХ I ЕКОНОМ1ЧНИХ Ц1ЛЕЙ У КОНТЕКСТ ФОРМУВАННЯ ХАРАКТЕРИСТИК
СТИГЛИХ Л1С1В
Дослщження, нещодавно проведенi у США та Канад^ присвяченi методам ста-лого ведення люового господарства, якi тдтримують бiологiчну рiзноманiтнiсть i функцiонування екосистем у люових ландшафтах, охоплених господарською дiяль-нiстю. Цi дослiдження торкаються питань ведення лiсового господарства в регюш поширення пiвнiчних твердолистяних лiсiв, структура яких сьогоднi значно прость ша, нiж та, яка була в умовах до европейського поселення. Одшею з можливостей п збагачення е модифшащя лiсiвничих методiв, як використовують для формування рiзновiкових деревосташв шляхом кращого врахування збурювальних впливiв, при-таманних природним процесам.
Неперевiрена ще гiпотеза полягае в тому, що цi тдходи прискорять темпи пiзньо-сукцесiйного розвитку i вiдповiднi екологiчнi функцп, а також забезпечать економiчну ефективнiсть лiсозаготiвель низько'' iнтенсивностi. Ми тестуемо цю ппо-тезу, використовуючи варiант рiзновiкового деревостану, вживаючи термiн "збагачення структурно'' комплексносп" (SCE), який описуе структуры характеристики стиглого деревостану. Порiвняно з двома традицшними системами менеджменту (вибiрковi рубання з вибором окремих дерев i групово-вибiрковi рубання), цей тдхщ модифiкований з метою пiдвищити здатшсть збереження власно'' структури насад-жень тсля проведення лiсозаготiвлi.
Дослiдження повторено у двох стиглих твердолистяних насадженнях твшчних лiсiв у штатi Вермонт (США). Дшянки, на яких здшснювали експерименти, i кон-трольнi дшянки займали площу 2 га. Заходи на дшянках рiзновiкових насаджень були повторенi двiчi. На дшянках iз доглядом SCE та контрольних дшянках - повторе-нi 4 рази. Структуры цiлi SCE охоплюють формування багатоярусного намету в насадженнях, збшьшення кiлькостi великих вщмерлих дерев i дерев, якi лежать на зем-лi, варiабельноi горизонтально'' повноти та перемщення до вищо'' ступенi дiаметра за сумою площ поперечних перетинiв дерев.
Даш щодо операцiйних витрат i доходiв упорядкованi в po3pi3i заходiв i про-дукцп пiд час i пiсля лiсозаготiвельних операцiй. Очшуваш чист прибутки були оць неш за кшькома варiантами витрат. Данi про структуру люу 3i6paHi за два роки до здшснення заходiв догляду за люом i три роки тсля ix проведення. Iмiтацiйне моде-лювання 50-рiчного розвитку насадження було здшснене з використанням двох iMi-тацiйниx моделей.
Виходячи з отриманих результат iмiтацiйного моделювання, можна прогнозу-вати iстотнi вiдмiнностi в розвитку насаджень. Шзносукцесшна структура буде роз-виватись швидше на вищому рiвнi SCE. Збiльшення чисельностi потомства великих дерев (з дiаметром > 50 см.) буде сповшьнене при проведенш традицiйниx заxодiв i отримае прискорення при заходах SCE. Лiсiвники повинш бути гнучкими, управля-ючи структурною комплексшстю насаджень iз застосуванням нетрадицшних тдхо-дiв. Здебiльшого менеджмент за системою SCE дасть нижчi доходи порiвняно з ш-тенсивнiшими методами люозаготвель. Звичайно, за умови наявносп вiдповiдниx лiсорослинниx умов i сприятливо'1' ситуацп на ринку, доходи можуть бути достатньо високими, аби покрити витрати на люозаготвельш роботи та отримати прибуток. Застосування пропозицш е досить широким - вщ вщтворення стиглих деревостанiв до ведення люового господарства з низькою iнтенсивнiстю.
Ключов1 слова: стале лiсове господарство, стигш лiси помiрноi зони, вщтво-рення, лiсiвничi методи формування рiзновiковиx деревосташв, пiвнiчнi твердолис-тянi лiси, розвиток люового насадження, економiка лiсозаготiвель.
William S. Keeton1; Austin R. Troy - Rubenstein School of Environment
and Natural Resources, University of Vermont
Balancing ecological and economic objectives while managing for old-growth forest characteristics
Recent research in the United States and Canada has focused on sustainable forestry practices that maintain biological diversity and ecosystem functioning across managed forest landscapes. In the northern hardwood region this includes managing for old-growth forest structure, which is vastly under-represented relative to pre-European settlement conditions. One possibility is to modify uneven-aged silvicultural practices to more closely approximate fine-scale natural disturbance effects. The as yet untested hypothesis is that these approaches would result in accelerated rates of late-successional development and related ecological functions, while also providing economic returns from low-intensity timber harvests. We are testing this hypothesis using a variant of uneven-aged forestry, termed "Structural Complexity Enhancement (SCE)," that promotes old-growth structural characteristics. This approach is compared against two conventional uneven-aged systems (single-tree selection and group-selection) modified to enhance post-harvest structural retention. The study is replicated at two mature, northern hardwood forests in Vermont, U.S.A. Manipulations and controls were applied to 2 ha units. The uneven-aged treatments were replicated twice; the SCE treatment and controls were each replicated four times. Structural objectives in SCE include multi-layered canopies, elevated large snag and downed log densities, variable horizontal density, and re-allocation of basal area to larger diameter classes. Data on operational expenses and revenue, sorted by treatment and product, were collected during and after logging operations. Expected net profits were evaluated under a number of different cost scenarios. Forest structure data have been collected over two years pretreatment and three years post-treatment. Fifty-year simulations of stand development were run using two forest development simulation models. There will be significant differences in stand development based on the simulation modeling. Late-successional structure will develop faster and to a greater degree under SCE. Large tree (>50 cm dbh) recruitment will be impaired under the conventional treatments, whereas recruitment will
1 [email protected]. Rubenstein School of Environment and Natural Resources, University of Vermont (Burlington, VT. 05405, U.S.A., (802) 656-2519)
be accelerated under SCE. Siviculturalists have the flexibility to manage for structural complexity using unconventional silvicultural approaches. Under most conditions these will result in lower economic returns compared to more intensive harvesting practices. However, given acceptable site quality and market conditions, they will provide sufficient economic returns to either offset the cost of harvesting or generate a profit. Applications range from old-growth forest restoration to low intensity timber management.
Keywords: sustainable forestry, old-growth temperate forests, restoration, uneven-aged silviculture, northern hardwood forests, forest stand development, timber harvest economics.
INTRODUCTION
Sustainable forestry practices across managed forest landscapes contribute to the maintenance of biological diversity and ecosystem functioning, for instance by providing connectivity between protected areas and unimpaired watershed processes. "Structure-" (Keeton 2005) or "natural disturbance-based" (Mitchell 2002, Seymour et al. 2002) silvicultural approaches provide alternatives for forest landscape management. Structure-based forestry focuses on the architecture of forest ecosystems at both stand-level and landscape-level spatial scales. Disturbance-based silviculture attempts to approximate the range of structural and compositional conditions associated with natural disturbance regimes. These approaches share the operational objective of managing for currently under-represented forest structures and age classes.
In the northern hardwood region of the northeastern United States and southeastern Canada a structure or disturbance-based approach would include managing for late-successional structure, which is vastly under-represented relative to pre-Eu-ropean settlement conditions (Cogbill 2000, Lorimer 2001, Lorimer and White 2003). An untested hypothesis is that silviculatural practices can accelerate rates of late-successional forest stand development (Franklin et al. 2002), promote desired structural characteristics, and enhance associated ecosystem functions more than conventional systems. We are testing this hypothesis using an approach, termed "Structural Complexity Enhancement (SCE), that promotes old-growth characteristics while also providing opportunities for low-intensity timber harvest (Table 1).
Table 1. Structural objectives and the corresponding silvicultural techniques used to promote those attributes in Structural Complexity Enhancement
Structural Objective Silvicultural Technique
Multi-layered canopy ■ Modified single-tree selection timber harvest ■ Release advanced regeneration ■ Establish new tree cohort
Elevated large snag densities ■ Girdling of selected medium to large sized, low vigor trees
Elevated downed woody debris densities and volume ■ Felling or pulling over trees to create large downed logs and tip-up mounds
Variable horizontal density ■ Harvested trees clustered around "crown release trees" ■ Variable density marking and harvest
Re-allocation of basal area to larger diameter classes ■ Rotated sigmoid target diameter distribution ■ High target basal area (34 m2/ha.) ■ Maximum target tree size set at 90 cm dbh
Accelerated growth in largest trees ■ Full and partial crown release of largest, healthiest trees
SCE is compared against two conventional uneven-aged systems advocated regionally for sustainable forestry (Mladenoff and Pastor 1993, Nyland 1998). Conventional uneven-aged prescriptions employed in this study are modified to increase post-harvest structural retention. In addition, group-selection treatments are modified to approximate the average canopy opening size associated with fine-scale natural disturbance events in the northeastern United States, based on the findings of Seymour et al. (2002).
Alternative Silvicultural Approaches
Interest in structure-based silviculture has evolved from studies of old-growth northern hardwood and mixed hardwood-conifer forests. These have demonstrated the ecological significance of specific structural elements associated with late-successional and old-growth forests (e.g. Tyrell and Crow 1994 b, Dahir and Lorimer 1996, McGee et al. 1999, Ziegler 2000). Availability of these structures can be highly limited in forests managed under conventional even and uneven-aged systems (Gore and Patterson 1985, McGee et al. 1999). Managing for late-successional forests has the potential to enhance ecosystem services associated with structural complexity, such as a subset of wildlife habitats, carbon storage, and riparian functions (Keeton et al. 2005). As a result, managing for old-growth structural characteristics, either in part or in full, is a proposed alternative silvicul-tural approach (Keddy and Drummond 1996, Lorimer and White 2003). While there has been much discussion of old-growth forest restoration in the theoretical literature (Trombulak 1996), there have been few experimental studies of relevant sil-vicultural methods for northern hardwood forests. Thus, it remains uncertain whether active restoration offers advantages over passive (or non-manipulative) restoration as means for recovering old-growth forest conditions. Our experimental test of SCE addresses this uncertainty.
The objectives of SCE include vertically differentiated canopies, elevated large dead tree and downed coarse woody debris (CWD) densities, variable horizontal density (including small gaps), and re-allocation of basal area to larger diameter classes (Table 1). The later objective is achieved, in part, using an unconventional marking guide based on a rotated sigmoid target diameter distribution. Rotated sig-moid diameter distributions have been widely discussed in the theoretical literature (O'Hara 1998), but their silvicultural utility has not been field tested. Sigmoidal form is one of several possible distributions in eastern U.S. old-growth forests (Leak 1996 and 2002, Goodburn and Lorimer 1999). These vary with disturbance history, species composition, and competitive dynamics. The distribution offers advantages for late-successional structural management because it allocates more growing space and basal area to larger trees. We predict that the rotated sigmoid distribution is sustainable in terms of recruitment, growth, and yield. If so, it would suggest that silvi-culturalists have greater flexibility in managing stand structure, biodiversity, and other ecosystem functions in the northern forest region than previously recognized.
Economic Tradeoffs
Our on-going research is evaluating the economic tradeoffs among the structure-based systems tested. The objective is to determine the stand (timber volume and quality), site (accessibility and cost of harvesting operations) and market
conditions necessary for structure-based systems to be economically viable and profitable. The economics of systems that promote structural complexity are poorly understood. Previous research has shown that revenue and product type vary widely with even small modifications to uneven-aged prescriptions (Niess and Strong 1992, Buongiorno et al. 1994). For alternative silvicultural approaches to have appeal for landowners and forest managers, their operational and economic feasibility must be demonstrated. We are evaluating economic feasibility from a present value framework, factoring in the price uncertainty stemming from the output of diverse products. This analysis will allow us to address a number of research questions. For instance, what are the economic tradeoffs involved with varying intensities of timber removal versus habitat enhancement? What is the economic viability of alternative silvicultural models under different scales of production? What is the level of economic uncertainty of these systems? How sensitive are returns to market prices of different forest products? And finally, what factors beyond stumpage volume, price, and interest rates affect economic feasibility and risk?
METHODS
Experimental Design and Data Collection
The study is replicated at two mature, multi-aged, northern hardwood forests in the northern Green Mountain Range in Vermont, U.S.A. Dominant overstory species include Acer saccharum (sugar maple), Betula alleghaniensis (yellow birch), and Fagus grandifolia (American beech). There are co-dominant or minor components of Tsuga canadensis (eastern hemlock), Acer rubrum (red maple), Picea rubens (red spruce), and Quercus rubra (red oak) at some sites.
There are three experimental manipulations. The first two are conventional uneven-aged systems (single-tree selection and group-selection) modified to increase post-harvest structural retention and to represent best available practices. Prescriptions are based on a target residual basal area of 18.4 m /ha, max. diameter of 60 cm, and q-factor of 1.3. Group-selection cutting patches are each approximately 0.05 ha in size. The third treatment is Structural Complexity Enhancement (SCE). The marking guide is based on a rotated sigmoid target diameter distribution applied as a non-constant q-factor. The marking guide is also derived from a target basal area (34 m /ha.) and maximum diameter at breast height (90 cm) indicative of old-growth structure. Accelerated growth in larger trees is promoted through full (4 or 3-sided) and partial (2-sided) crown release. Prescriptions for enhancing dead tree and downed woody debris volume and density are based on pre-har-vest CWD volume and literature-derived targets. On one SCE unit at each of the two study areas, downed logs are created by pulling trees over, rather than felling, to create pits and exposed root wads.
Each of the first two treatments (uneven-aged) is replicated twice; the third (SCE) is replicated four times. Two un-manipulated control units are located at each of the two study areas. Treatment units are 2 ha in size and separated by 50 meter (min.) buffers. Treatments were randomly assigned. Experimental manipulations (i.e. logging) were conducted on frozen ground in winter 2003. Sample data were collected from five 0.1 ha permanent sampling plots randomly established in
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each treatment unit. Forest structure data, including leaf area index (LAI), detailed measurements of individual trees, and coarse woody debris (CWD) densities and volumes, have been collected over two years pretreatment and three years post-treatment. Plots were stem-mapped using an integrated laser range finder and digital electronic compass.
To track operational expenses loggers were required to file daily worksheets. These recorded hours worked, equipment use and repairs, number and type of loads, and work conditions. Harvested logs were separated into four product grades: saw logs, veneer logs, firewood, and chip wood. Logs were then segregated by treatment, transported, and tracked independently by unit through to scaling (valuation) at the processing mill. In this way harvest volumes and revenue could be tracked by treatment, species, size class, and grade or product.
Data Analysis
Forest sample data were used as inputs for 3-dimensional modeling in the Stand Visualization System (SVS) (McGaughey 1997). The Northeast Decision Model (Twery et al. 2005) was used to generate stand structure metrics based on pre and post-harvest sample data. Structural metrics were analyzed using a before/after/control/impact statistical approach. For this purpose we used Tukey-tests, Analysis of Variance, and post-hoc Bonferroni or Least Significant Difference multiple comparisons. Fifty-year simulations of stand development were run using two models: the northeastern variant of the Forest Vegetation Simulator (FVS) and NE-TWIGS (Bush 1995). The FVS modeling structure is based on NE-TWIGS, which is an individual tree-based, distance-independent stand growth simulator. However, mortality and large-tree growth functions operate slightly differently in NE-FVS and calculations are made every ten years, rather on the annual time step employed in NE-TWIGS. We ran simulations for individual experimental units and for both "no-treatment" (pre-harvest data) and "treatment" (post-harvest data) scenarios. Cumulative basal area increment (CBAI) was calculated for each simulation run at 5 year intervals. Projections were normalized on a unit by unit basis by calculating the differences between "no-treatment" and "treatment" scenarios at each time step. The Kol-mogorov-Smirnov two-sample goodness of fit test was used to test for differences between treatment groups along mean projected time series.
Economic data were entered into a Microsoft Access relational database. The series of linked tables includes hours worked by activity and treatment unit; receipts and quantity by product grade, type and load; number, size and destination of loads by treatment unit; costs per hour by equipment and personnel class, and added expenses. Operational expense and revenue data were used to quantify costs and net profits under each treatment for three cost scenarios designated as "nonprofit," "semi-profit," and "for-profit." The scenarios reflected the extent to which expenses not directly linked to harvesting, such as timber marking, would be accounted for as costs. Linear regression analysis was used to model relationships between receipts and pre-harvest timber volume. Revenue by treatment was also evaluated as a ratio relative to labor hours, since this was the greatest operational expense by at least one order of magnitude.
RESULTS
Residual stand structure
Visualizations generated in SVS illustrate the high degree of structural complexity maintained by both SCE and single-tree selection (figure 1). Post-harvest basal area, relative density, canopy closure, and LAI were significantly (a = = 0.05) higher under SCE compared to conventional treatments. Conventional treatments resulted in significantly lower aboveground biomass (P = 0.014), total basal area (P = 0.003), relative density (P = 0.002), and stem density (P = 0.008) in comparison to control units. SCE did not result in statistically significant contrasts with controls. Canopy closure was most variable across group-selection units. There were significant differences (P < 0.001) in LAI responses among treatments. Single-tree and group selection cuts reduced LAI by 19.8 and 29.9 % respectively. LAI reductions were lowest in SCE units (9.4 %o), indicating high retention of vertical complexity. LAI was significantly more spatially variable for both SCE (P = 0.031) and group-selection (P = 0.010) compared to single tree selection; within-treatment variance was not significantly different between SCE and group-selection units (P = = 0.296). These results are indicative of the high degree of horizontal structural variability expected for both group-selection and SCE, achieved in the later through variable density marking and clustered harvesting around crown-release trees. SCE shifted residual diameter distributions to a form statistically indistinguishable (a = = 0.05) from the target rotated sigmoid form.
Crown release and vertical development
Variable density timber harvesting was used successfully to crown release 45 dominant trees per ha. on average in SCE units. When combined with the average pre-treatment number (20 per ha) of large trees (> 50 cm dbh), this exceeds our future target of 55 large trees per ha. The excess provides a "margin of safety" to accommodate canopy mortality. Crown release is likely to accelerate growth rates in the affected dominant trees by 50 % or more based on previous modeling (e.g. Singer and Lorimer 1997). Crown release also resulted in spatial aggregations of harvested trees, creating canopy openings and variable tree densities. Elevated light availability associated with this effect is likely to promote vertical differentiation of the canopy through release and regeneration effects.
Coarse woody debris enhancement
SCE prescriptions resulted in substantially elevated densities and volumes of both downed coarse woody debris and standing snags (dead trees).
The structural complexity enhancement treatments increased coarse woody debris (> 30 cm dbh) densities, on average, by 10 boles/ha for snags and 12 boles/ha for downed logs. Snags were created primarily by girdling diseased, dying, or poorly formed trees.
Pulling trees over was successful in most cases at creating large exposed root wads and pits. There were statistically significant differences (P = 0.002) between treatments with respect to downed CWD recruitment. Post-harvest CWD (logs > 10 cm diameter) volumes were 140 % higher on average than pre-harvest levels in SCE units; mean CWD volume increased 30 % in conventional uneven-aged units due to residual slash.
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Single-Tree Selection Unit
Stand Visualization System_UNIT 4 PREHARVEST PLOT.SVS
Stand Visualization System
UNIT 4 POST HARVEST PLOT.SVS
Structural Complexity Enhancement Unit
Stand Visualization System UNIT 3 PREHARVEST PLOT.SVS Stand Visualization System UNIT ! POST HARVEST PLOT.SVS
Figure 1. Output of the Stand Visualization System (SVS) contrasting a single-tree selection unit (above) and a Structural Complexity Enhancement (SCE) unit (below) at the Mount Mansfield study area. Shown are images of pre- and post-harvest stand structure for 1 ha. blocks. Shaded circles represent tree crowns (with species-specific coloration) seen from a simulated aerial view. Note the high degree of post-harvest structure (e.g. basal area and stem density), canopy closure, vertical complexity, and downed log densities in the SCE unit. Note the similar, though lower, degree of structural
retention in the single-tree selection unit
Projected stand development
Stand development projections suggest that total basal area under SCE will, on average, approach 34 m /ha after 50 years of development (figure 2). This is 24 % (or 8 m /ha) higher than the mean predicted for the conventional uneven-aged units. Projected basal area for SCE also exceeds the mean predicted for control units by 13 % (or 4.5 m /ha). Conventional units were projected to have basal areas still 12 % (or 3.6 m2/ha) below the control units after 50 years of development. However, the difference among treatments is largely an artifact of the higher residual basal area left by SCE. The projections showed no significant differences in absolute growth rates between treatment scenarios. Both SCE (P < 0.05) and conventional treatments (P < 0.01) are projected to significantly accelerate tree growth rates above that expected with no treatment. However, when projected development is normalized against the null scenario (development expected with no treatment), the simulations indicate that conventional systems will increase cumu-
lative basal area increment (CBAI) slightly more, although this difference was not statistically significant. Aboveground biomass production is accelerated 5.1 % for SCE and 1.9 % for conventional treatments compared to no treatment scenarios.
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Figure 2. Results of stand development simulation modelling. Shown are 50 year projections of post-treatment cumulative basal area production (live trees only).
Error bars are ±1 standard error of the mean
Neither SCE nor conventional treatments resulted in projected basal area or biomass values that exceeded those projected for "no treatment" scenarios. However, basal area in SCE units recovered to within 89 % of the no-treatment scenario, whereas conventional units recovered to within 77 %o on average. After 50 years SCE results in aboveground biomass that is 91.4 %o of that projected under no treatment, while the conventional treatments result in 79.1 % of the no treatment potential. These differences were statistically significant (a = 0.05).
SCE is projected to enhance rates of large tree recruitment over no treatment scenarios (figure 3). There will be an average of 5 more large trees (> 50 cm dbh) per ha than there would have been without treatment after 50 years in SCE units. There will be 10 fewer large trees/ha on average in the conventional units than would have developed in the absence of timber harvesting. Projections suggest that a rotated sigmoid diameter distribution will be sustained over 50 years in SCE units. The corresponding projected basal area distributions indicate significant reallocation of basal area and biomass into the largest size classes (e.g. > 50 cm dbh) for SCE.
Economic return
Profit margins were highly variable between units due to differences in site quality and treatment prescriptions. Pre-harvest timber volume (and associated surplus available for harvest) was strongly related to revenue for both SCE (r2 = 0.56) and conventional treatments (r2 = 0.71). Thus, site quality accounted for the most variability in harvest revenue among similarly treated units. Single-tree selection resulted in the highest net profits, but was also randomly assigned to units
with the highest pre-treatment volumes and greater harvesting opportunities. Average revenue generated per ha. for this treatment was $4,150. Group-selection provided a moderate profit margin but, under a "for profit" scenario, incurred a deficit in one unit where pre-harvest volume was low. Group-selection generated an average revenue of $2,930 per ha. SCE revenues ranked lowest among the treatments, producing $1,710 per ha.
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UnevEn-ag
Figure 3. Projected change in large tree densities after 50years. Values represent the difference between treatment and no treatment scenarios. Note the increased recruitment of large trees under SCE versus the impairment of large tree recruitment under the conventional uneven-aged treatments. Error bars are ±1 standard error of the mean
Net profits for SCE varied by treatment unit and cost scenario. Under a "for profit" scenario, net profits for SCE were only positive for sites with higher pre-treatment timber quality. If marking and incidental costs were not considered, SCE resulted in a net profit for all but one unit, where a $425/ha. deficit was incurred. Under favorable cost and site-quality (e.g. top two highest grossing units per treatment) scenarios, net profits per ha. for single-tree selection, group selection, and SCE were $1,550, $900, and $300 respectively. Since these margins include labor costs, they represent profits that would be returned to a landowner after logging contractors have been paid. Sensitivity analysis showed that reducing labor costs by even a small amount would result in profitability for marginal SCE units under all cost scenarios.
Assessing revenues as a ratio relative to the number of labor hours necessary to conduct a treatment provides a different picture of economic feasibility. Whether a silvicultural approach is more expensive to implement is a critical question. When we control for differences in operability between sites, SCE had a revenue to labor ratio that was 81 % of the average for conventional treatments (figure 4). From this standpoint SCE may result in a small increase in labor costs relative to conventional treatments.
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Figure 4. Mean ratio of before trucking receipts (in U.S. dollars) to labour hours for each of the three experimental treatments. Differences were not statistically significant (P> 0.05). Error bars are ±1 standard error of the mean
DISCUSSION
Silvicultural techniques can be used effectively to promote old-growth structural characteristics in northern hardwood and mixed northern hardwood-conifer forests. Both the uneven-aged and structural complexity enhancement (SCE) systems tested maintain high levels of post-harvest structure and canopy cover. These are indicative of lower intensity, minimal impact forestry practices (Franklin et al. 2002, McEvoy 2004). However, SCE maintains, enhances, or accelerates develop of CWD, canopy layering, overstory biomass, large tree recruitment, and other structural attributes to a greater degree. In addition, SCE results in a rotated sigmoid diameter distribution that appears sustainable at least over 50 years, and consequently reallocates growing space and aboveground structure into larger size classes. This contributes to enhanced large tree structure, foliage biomass, and associated canopy complexity.
Both SCE and conventional uneven-aged treatments will result in accelerated tree growth rates according to model projections. Since the conventional treatments had significantly lower residual basal areas, this result is consistent with previous research on growth responses to stocking density in northern hardwoods (Leak et al. 1987). However, an important effect of SCE is the promotion of large tree recruitment, whereas this process is impaired under conventional treatments that include maximum diameter limits. Projected basal area and biomass are also higher after 50 years of development under SCE due to greater post-harvest structural retention. However, none of treatments are likely to develop basal areas or abovegro-und biomass exceeding levels that would have accumulated without treatment. Passive restoration may ultimately develop higher levels of these characteristics. However, that conclusion does not account for the accelerated rates of large tree recruitment, reallocation of basal area, and associated structural complexity projected for SCE. Active restorative approaches thus offer advantages with respect to development of canopy complexity and large tree structure.
SCE resulted in significantly elevated CWD densities and volumes. However, it remains uncertain whether this effect will persist until natural recruitment rates increase, or, alternatively, whether CWD enhancement in mature stands has only transient or short-term management applications. Most of the newly added CWD is un-decayed. It is likely than decay class distributions will shift over time towards well-decayed material. As time passes, this will render silviculturally enhanced CWD increasingly available as habitat and as a nutrient source (Tyrrell and Crow 1994 a).
SCE may have a variety of useful applications, ranging from old-growth restoration, to riparian management, to low-intensity timber management and wildlife habitat enhancement. However, the degree of implementation and the number of stand entries will vary by application (Table 2). Management application will also depend greatly on economic feasibility under a variety of site quality, product, and market conditions (Niese and Strong 1992). The relatively low returns for SCE suggest that its feasibility is highly sensitive to site quality and market conditions. Where these are poor, SCE will at best cover expenses and at worst may result in a deficit. These scenarios might be acceptable in a limited number of settings, for instance where restoration funds are available for nature preserve management, or where access roads, landings, and skid trails are already paid for. Where a positive profit margin is a required, SCE would only be marketable where site quality (e.g. volume) is high and market conditions (e.g. lumber prices, fuel costs, and interest rates) are favorable. Under these conditions SCE offers an alternative that provides revenue from low-intensity harvest while also meeting ecological management objectives. Conventional uneven-aged approaches are also clearly sensitive to site quality and market conditions, and can result in deficits where these are poor. Conventional approaches are more robust economically in comparison to SCE only because harvest volumes are moderately higher on a per unit area basis, resulting in economies of scale. Both and SCE and conventional approaches would likely manifest economies of scale as treatment area increases due to lower cost to revenue ratios. Thus, SCE may be more feasible at larger scales of application.
Forest managers have the flexibility to manage for a wide range of structural characteristics and associated ecosystem functions. Uneven-aged systems provide some but not all of these or provide them to a more limited extent. Maximum diameter limits significantly retard the potential for large tree (live and dead) recruitment based on the results. Stand development is thus continuously truncated by multiple uneven-aged cutting entries. The results show that SCE's marking guide can be used to successfully achieve a rotated sigmoid diameter distribution. Unconventional prescriptive diameter distributions, such as the rotated sigmoid, combined with higher levels of residual basal area, very large (or no) maximum diameters, and crown release are alternatives for retaining high levels of post-harvest structure and for promoting accelerated stand development.
Forest managers have the flexibility to manage for a wide range of structural characteristics and associated ecosystem functions. Uneven-aged systems provide some but not all of these or provide them to a more limited extent. Maximum diameter limits significantly retard the potential for large tree (live and dead) recru-
itment based on the results. Stand development is thus continuously truncated by multiple uneven-aged cutting entries. The results show that SCE's marking guide can be used to successfully achieve a rotated sigmoid diameter distribution. Unconventional prescriptive diameter distributions, such as the rotated sigmoid, combined with higher levels of residual basal area, very large (or no) maximum diameters, and crown release are alternatives for retaining high levels of post-harvest structure and for promoting accelerated stand development.
Table 2. Potential applications of SCE as an approach for incorporating old-growth
structure into managed forests
Application # Entries Late-Successional Structural Development
Old-growth One or possibly High
promotion two entries
Riparian Single or multiple Moderate to high
management
Timber emphasis Multiple Low to moderate
Acknowledgements
This research was supported by grants from the USDA CSREES National Research Initiative, the Vermont Monitoring Cooperative, the Northeastern States Research Cooperative, and the USDA McIntire-Stennis Forest Research Program.
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